Actuator

ABSTRACT

An actuator  100  includes: a pair of driving portions  1, 11  spaced apart from each other; a movable portion  2  provided between the pair of driving portions  1, 11 ; a pair of supporting portions  3, 3  for supporting the pair of driving portions  1, 11  and the movable portion  2 ; a pair of first elastic connecting portions  4, 4  which respectively connect the pair of driving portions  1, 11  to the pair of supporting portions  3, 3  so that each of the driving portions  1, 11  can rotate with respect to the supporting portions  3, 3 ; and a pair of second elastic connecting portions  5, 5  which respectively connect the movable portion  2  to the pair of driving portions  1, 11  so that the movable portion  2  can rotate in accordance with the rotation of the pair of driving portions  1, 11 . In this case, each of the pair of driving portions  1, 11  rotates around a rotation central axis  41  of the actuator  100  by application of an alternating voltage, whereby the movable portion  2  rotates around the rotation central axis  41  of the actuator  100 . Further, the torsional rigidity of each of the first elastic connecting portions  4, 4  is larger than that of each of the second elastic connecting portions  5, 5.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No.2004-363608 filed Dec. 15, 2004, which is hereby expressly incorporatedby reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an actuator, and in particular relatesto an actuator of the type employing a two-degree-of-freedom vibrationsystem.

2. Description of the Prior Art

There is known a polygon mirror (rotary polyhedron) as an actuatorprovided in laser printers, for example. In such a printer, in order toachieve higher-resolution and higher-quality printed output as well ashigher-speed printing, it is necessary to rotate the polygon mirror athigher speed. Currently, an air bearing is used to rotate the polygonmirror at high speed with stability. However, there is a problem in thatit is difficult to rotate the polygon mirror at much higher speed thanthe speed available at the present. Further, although a larger motor isrequired in order to rotate the polygon mirror at higher speed, use ofsuch a larger motor gives rise to a problem in that it is difficult tominiaturize the size of an apparatus in which the polygon mirror isused. Furthermore, use of such a polygon mirror gives rise to anotherproblem in that the structure of the apparatus becomes necessarilycomplicated, thus leading to increased manufacturing cost.

On the other hand, a single-degree-of-freedom torsional vibrator asshown in FIG. 8 has been proposed since the early stages of research inthe field of actuators. Since this vibrator uses flat electrodes whichare arranged in parallel with each other, it can have quite simplestructure (see K. E. Petersen: “Silicon Torsional Scanning mirror”,IBMJ. Res. Develop., Vol. 24 (1980), P. 631, for example). Further, asingle-degree-of-freedom electrostatic drive type vibrator obtained bymodifying the torsional vibrator described above so as to have acantilever structure has also been proposed (see Kawamura et al.“Research in micromechanics using Si”, Proceedings of the Japan Societyfor Precision Engineering Autumn Conference (1986), P. 753, forexample).

FIG. 8 shows such a single-degree-of-freedom electrostatic drive typetorsional vibrator. In the torsional vibrator shown in FIG. 8, a movableelectrode plate 1300 made of monocrystalline silicon is fixed at endfixing portions 1300 a thereof to the both ends of a glass substrate1000 through spacers 1200. The movable electrode plate 1300 includes amovable electrode portion 1300 c which is supported by the end fixingportions 1300 a through narrow torsion bars 1300 b. Further, a fixedelectrode 1400 is provided on the glass substrate 1000 so as to beopposed to the movable electrode portion 1300 c through a predeterminedelectrode interval. Specifically, the fixed electrode 1400 is arrangedin parallel with the movable electrode portion 1300 c through theelectrode interval therebetween. The fixed electrode 1400 is connectedto the movable electrode plate 1300 via a switch 1600 and a power source1500.

In the torsional vibrator having the structure described above, when avoltage is applied across the movable electrode portion 1300 c and thefixed electrode 1400, the movable electrode portion 1300 c rotatesaround the axis of the torsion bars 1300 b due to electrostaticattraction. Since electrostatic attraction is inversely proportional tothe square of an electrode interval, it is preferable for this type ofelectrostatic actuator to have a small electrode interval between themovable electrode portion 1300 c and the fixed electrode 1400. However,in such a single-degree-of-freedom torsional vibrator described above,the movable electrode portion 1300 c which serves as a movable portionis also provided with the electrode. Therefore, if the electrodeinterval becomes too small, a movable range (rotational angle) of themovable electrode portion 1300 c is necessarily limited. On the otherhand, in order to enlarge the movable range of the movable electrodeportion 1300 c, it is necessary to widen the electrode interval and thisin turn needs a large driving voltage. Namely, such asingle-degree-of-freedom torsional vibrator described above involves aproblem in that it is difficult to achieve both of low-voltage drivingand large displacement.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an actuatorcapable of driving with a large rotational angle or large deflectionangle.

In order to achieve the object, the invention is directed to anactuator. The actuator of the invention includes:

a pair of driving portions spaced with each other;

a movable portion provided between the pair of driving portions;

a pair of supporting portions for supporting the pair of drivingportions and the movable portion;

a pair of first elastic connecting portions which respectively connectthe pair of driving portions to the pair of supporting portions so thateach of the driving portions can rotate with respect to the supportingportions; and

a pair of second elastic connecting portions which respectively connectthe movable portion to the pair of driving portions so that the movableportion can rotate in accordance with the rotation of the pair ofdriving portions,

wherein each of the pair of driving portions is rotated around arotation central axis of the actuator by application of an alternatingvoltage, whereby the movable portion rotate around the rotation centralaxis of the actuator, and

wherein the torsional rigidity of each of the first elastic connectingportions is larger than that of each of the second elastic connectingportions.

This makes it possible to enlarge a rotational angle (deflection angle)of the movable portion. In addition, since movable portion is providedbetween the pair of driving portions, it is possible to operate themovable portion with a larger torque.

In the actuator according to the invention, it is preferred that thesecond moment of area of each of the pair of first elastic connectingportions is larger than that of each of the second elastic connectingportions.

This makes it possible to enlarge a rotational angle (deflection angle)of the movable portion.

In the actuator according to the invention, it is preferred that each ofthe pair of first elastic connecting portions is constructed from twoconnecting members each spaced from the rotation central axis with apredetermined distance.

This makes it possible to enlarge a rotational angle (deflection angle)of the movable portion. In addition, it is possible to stabilize thedriving (rotational operation) of the driving portion.

In the actuator according to the invention, it is preferred that each ofthe pair of driving portions is constructed so as to rotate around therotation central axis by making each of the two connecting membersbending deformation.

This makes it possible to enlarge a rotational angle (deflection angle)of the movable portion. In addition, it is possible to stabilize thedriving (rotational operation) of the driving portion.

In the actuator according to the invention, it is preferred that each ofthe pair of second elastic connecting portions is coaxially providedwith respect to the rotation central axis.

This makes it possible for the movable portion to rotate stably.

In the actuator according to the invention, it is preferred that each ofthe pair of driving portions is driven by means of Coulomb forcegenerated by the application of the alternating voltage.

This makes it possible to enlarge a rotational angle (deflection angle)of the movable portion.

In the actuator according to the invention, it is preferred that theactuator further includes:

a counter substrate provided so as to be spaced apart from the pair ofdriving portions with a predetermined distance; and

a plurality of electrodes respectively provided at the positions on thecounter substrate corresponding to those of the pair of drivingportions,

wherein each of the pair of driving portions is driven by means ofCoulomb force generated by applying the alternating voltage between eachof the plurality of electrodes and the corresponding driving portion.

This makes it possible for the driving portion to drive stably.

In the actuator according to the invention, it is preferred that theplurality of electrodes include two pairs of electrodes, and each pairof the two pairs of electrodes is provided at the positions on thecounter substrate which correspond to both ends of the correspondingdriving portion in the direction substantially perpendicular to therotation central axis.

This makes it possible for the driving portion to drive more stably.

In the actuator according to the invention, it is preferred that thecounter substrate has an opening at the portion corresponding to themovable portion.

Thus, it is possible to set the rotational angle (deflection angle) ofthe movable portion.

In the actuator according to the invention, it is preferred that thepair of supporting portions, the movable portion, the pair of drivingportions, the pair of first elastic connecting portions and the pair ofsecond elastic connecting portions are formed by subjecting a basematerial to an etching process.

This makes it possible to form (obtain) the supporting portions, themovable portion, the driving portions, the first elastic connectingportions, and the second elastic connecting portions.

In the actuator according to the invention, it is preferred that thebase material is formed of silicon.

This makes it possible to form such a structure having conductivityeasily. In addition, it is possible to obtain the movable portion thatcan drive stably.

In the actuator according to the invention, it is preferred that theactuator further includes a light reflecting portion provided on themovable portion.

Although the actuator of the invention can be applied to various typesof apparatuses, it is preferable that the actuator of the invention isapplied to an optical scanner, for example. In such a case, it ispossible to change a light path of the light emitted to the actuatorprovided with the light reflecting portion.

In the actuator according to the invention, it is preferred that theactuator is of the type which employs a two-degree-of-freedom vibrationsystem, and the frequency of the alternating voltage is set so as to besubstantially the same as a lower resonance frequency of resonancefrequencies of the two-degree-of-freedom vibration system at which thepair of driving portions and the movable portion resonate.

Thus, it is possible to enlarge the rotational angle (deflection angle)of the movable portion while preventing the amplitude of each of thedriving portions.

In the actuator according to the invention, it is preferred that atleast one of the pair of first elastic connecting portions and the pairof second elastic connecting portions includes a piezoresistive element.

Thus, it is possible to detect the rotational angle and rotationalfrequency of the movable portion, for example. Further, it is possibleto use the detected results for control of the posture of the movableportion.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will become more readily apparent from the following detaileddescription of preferred embodiment of the invention which proceeds withreference to the appending drawings.

FIG. 1 is a plan view which shows an embodiment of the actuatoraccording to the invention.

FIG. 2 is a cross-sectional view taken along line A-A in FIG. 1.

FIG. 3 is a plan view which shows the arrangement of the electrodes ofthe actuator shown in FIG. 1.

FIG. 4 is a drawing which shows an example of the alternating voltage tobe applied to the actuator shown in FIG. 1.

FIG. 5 is a graph which shows the frequency of an alternating voltageapplied and the resonance curves of the driving portions and the movableportion.

FIG. 6 is a drawing for explaining a method of manufacturing theactuator according to the present embodiment.

FIG. 7 is a plan view which shows an alternative example of the firstelastic connecting portion.

FIG. 8 is a perspective view which shows a conventional actuator.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a preferred embodiment of an actuator according to theinvention will be described with reference to the appended drawings.

First, an embodiment of the actuator according to the invention will bedescribed. FIG. 1 is a plan view which shows an embodiment of theactuator according to the invention. FIG. 2 is a cross-sectional viewtaken along line A-A in FIG. 1. FIG. 3 is a plan view which shows thearrangement of the electrodes of the actuator-shown in FIG. 1. FIG. 4 isa drawing which shows an example of the alternating voltage to beapplied to the actuator shown in FIG. 1. FIG. 5 is a graph which showsthe frequency of an alternating voltage applied and the resonance curvesof the driving portions and the movable portion. In the followingdescription using FIGS. 1 and 3, for convenience of description, it isto be noted that the upper side, the lower side, the right side and theleft side in FIGS. 1 and 3 will be referred to as the “upper side”,“lower side”, “right side” and the “left side”, respectively.

An actuator 100 shown in FIGS. 1 and 2 includes a pair of drivingportions 1 and 11, a movable portion 2, and a pair of supportingportions 3. In the actuator 100, the movable portion 2 is positioned atthe center thereof, and the driving portions 1, 11 are provided at oneend side (right side in FIGS. 1 and 2) and the other side (left side inFIGS. 1 and 2) of the movable portion 2, respectively. Each of thedriving portions 1, 11 and the movable portion 2 has a substantiallyplate shape.

Further, one supporting portion 3 is arranged at the right side of thedriving portion 1 in FIGS. 1 and 2, while the other supporting portion 3is arranged at the left side of the driving portion 11 in FIGS. 1 and 2.In the present embodiment, as shown in FIGS. 1 and 2, the drivingportions 1, 11 have substantially the same shape and size as each other,and are symmetrically provided at the both ends of the movable portion2. On the upper surface of the movable portion 2 of the presentembodiment (that is, the surface of the movable portion 2 opposite tothe surface that faces a counter substrate 6 which will be describedlater), there is provided a light reflecting portion 21.

Further, as shown in FIG. 1, the actuator 100 includes a pair of firstelastic connecting portions 4, 4 and a pair of second elastic connectingportions 5, 5. The pair of first elastic connecting portions 4, 4connect the driving portions 1, 11 to the supporting portions 3, 3,respectively, so that each of the driving portions 1, 11 can rotate withrespect to the corresponding supporting portion 3. The pair of secondelastic connecting portions 5, 5 connect the movable portion 2 to thedriving portions 1, 11, respectively, so that the movable portion 2 canrotate in accordance with the rotation of the driving portions 1, 11.

In other words, the movable portion 2 is connected to the drivingportions 1, 11 via the second elastic connecting portions 5, 5,respectively, and the driving portions 1, 11 are connected to thesupporting portions 3, 3 via the first elastic connecting portions 4, 4,respectively. In this regard, the pair of second elastic connectingportions 5, 5 has substantially the same shape and size as each other.In this case, the rotation central axis of each of the first elasticconnecting portions 4, 4 and the rotation central axis of each of thesecond elastic connecting portions 5, 5 are coaxially provided toconstitute a central axis for the rotation of the driving portions 1, 11and the movable portion 2 (that is, rotational axis) 41.

Each of the first elastic connecting portions 4, 4 is constructed fromtwo connecting members 42, 42 each spaced from the rotation central axis41 at even intervals (with a predetermined distance). The two connectingmembers 42, 42 have substantially the same shape and size as each other,and each of the connecting members 42, 42 is deformed so as to be mainlybent when the driving portions 1, 11 are driven.

Further, as shown in FIG. 1, in the case where the width of each of thesecond elastic connecting portions 5, 5 (the length between both ends ofeach of the second elastic connecting portions 5, 5 in a directionsubstantially perpendicular to the rotation central axis 41) is definedas a and the width of each of the two connecting members 42, 42 in thesame direction is defined as b, then the widths thereof are formed sothat a and b are substantially the same as each other.

Moreover, as shown in FIG. 2, in the case where the thickness of each ofthe first elastic connecting portions 4, 4 (the length thereof in theup-and-down direction) is defined as c and the thickness of each of thesecond elastic connecting portions 5, 5 in the same direction is definedas d, then the thicknesses thereof are formed so that c and d aresubstantially the same as each other.

Furthermore, in the case where the length of each of the connectingmembers 42 in the right-and-left direction in FIG. 1 is defined as e andthe length of each of the second elastic connecting portions 5, 5 in thesame direction is defined as f, then the lengths thereof are formed sothat e and f are substantially the same as each other.

The driving portions 1, 11, the movable portion 2, the supportingportions 3, 3, the first elastic connecting portions 4, 4, and thesecond elastic connecting portions 5, 5 are formed of, for example,silicon as a main material as will be described later, and it ispreferable that they are formed as one unit.

Further, as shown in FIG. 2, the actuator 100 of the present embodimentincludes the counter substrate 6. The counter substrate 6 is provided soas to be opposed to the driving portions 1, 11 and the movable portion2. The counter substrate 6 is formed of various glass materials orsilicon as a main material, for example, and is connected (supported) tothe supporting portions 3, 3.

In the counter substrate 6, as shown in FIGS. 2 and 3, an opening 61 isformed at a position corresponding to the position of the movableportion 2. The opening 61 functions as an escape portion (or reliefportion) for preventing the counter substrate 6 from being contact withthe movable portion 2 when the movable portion 2 rotates. By providingthe opening (escape portion) 61, it is possible to set the deflectionangle (amplitude) of the movable portion 2 to larger while preventingthe whole actuator 100 from being made larger.

In this regard, the escape portion does not need to be an opening thatopens at the lower surface of the counter substrate 6 (that is, thesurface opposite to the movable portion 2) as long as it has aconfiguration by which it is possible to achieve the effect describedabove efficiently. For example, the escape portion may be constitutedfrom a concave portion formed on the upper surface of the countersubstrate 6.

Further, in the present embodiment, as shown in FIGS. 2 and 3, on theupper surface of the counter substrate 6 (that is, the surface thatfaces the driving portions 1, 11), there are provided two pairs ofelectrodes 7 at the positions corresponding to the positions of both endportions of the driving portions 1, 11, respectively. The two pairs ofelectrodes 7 are provided so that two electrodes 7 in each pair ofelectrodes 7 become substantially symmetrical to each other with respectto the surface that includes the rotation central axis 41 (that is, therotational axis of the driving portions 1, 11) and is perpendicular tothe counter substrate 6.

The two pairs of electrodes 7 are respectively connected to the drivingportions 1, 11 via a power source (not shown in the drawings) so that analternating voltage (driving voltage) can be applied across the drivingportions 1, 11 and the two pairs of electrodes 7. In this regard,insulating films (not shown in the drawings) are respectively providedon the surfaces of the driving portions 1, 11 (that is, the surfaces ofthe driving portions 1, 11 facing the electrodes 7). This makes itpossible to prevent a short circuit from occurring between the drivingportions 1, 11 and each of the electrodes 7.

The actuator 100 having the structure as described above constitutes atwo-degree-of-freedom vibration type actuator which has two vibrationsystem in which the driving portions 1, 11 and the first elasticconnecting portions 4, 4 constitute a first vibration system, and themovable portion 2 and the second elastic connecting portions 5, 5constitute a second vibration system.

Such an actuator 100 drives as follows. Namely, when a sinusoidal wave(alternating voltage) or the like, for example, is applied between eachof the driving portions 1, 11 and the corresponding electrodes 7, morespecifically, for example, when the driving portions 1, 11 are connectedto ground, and a voltage signal having a waveform of a single-phasehalf-wave rectification as shown in FIG. 4A is applied to the twoelectrodes 7, 7 at the upper side in FIG. 3 and a voltage signal havinga waveform as shown in FIG. 4B, which is out of phase with the waveformas shown in FIG. 4A by 180 degrees, is applied to the two electrodes 7,7 at the lower side in FIG. 3, Coulomb force (electrostatic force) isgenerated between the driving portions 1, 11 and the correspondingelectrodes 7, 7, respectively.

The intensity of the Coulomb force varies depending on the change in thephase of the sinusoidal wave. Each of the connecting members 42, 42 isdeformed so as to be mainly bent by means of the Coulomb force. Althoughbending stress is applied to each of the connecting members 42, 42(bending deformation is mainly generated in each of the connectingmembers 42, 42 by itself), torsional deformation with respect to therotation central axis 41 is mainly generated as the whole of the firstelastic connecting portions 4, 4 (first vibration system). This makesthe driving portions 1, 11 to rotate (vibrate) around the rotationcentral axis 41 (a central axis of the first elastic connecting portions4, 4).

Then, along with the vibration (driving) of the driving portions 1, 11,the second elastic connecting portions 5, 5 receives such torsionaldeformation to be torsionally deformed mainly. The movable portion 2connected to the driving portions 1, 11 via the second elasticconnecting portions 5, 5 also rotate (vibrate) around the rotationcentral axis 41 (of the second elastic connecting portions 5, 5).

Next, the relation of the second moment of area between the firstelastic connecting portions 4, 4 and the second elastic connectingportions 5, 5 to the torsional rigidity thereof will be explained usingthe first elastic connecting portion 4 and the second elastic connectingportion 5 at the right side of FIG. 1 typically.

The second moment of area between the first elastic connecting portion 4and the second elastic connecting portion 5 (coefficient ofhard-to-deform) is determined using the shape and cross-sectional areaof each of the first and second elastic connecting portions 4 and 5.

By setting the shape, arrangement and size of the first elasticconnecting portions 4, 4 and the second elastic connecting portions 5, 5to those as described above, the second moment of area of the firstelastic connecting portion 4 with respect to the rotation central axis41 can be set to a predetermined value larger than that of the secondelastic connecting portion 5.

On the other hand, the torsional rigidity of each of the first elasticconnecting portion 4 and second elastic connecting portion 5 isexpressed as the product of elastic modulus thereof and the secondmoment of area thereof, respectively.

By setting the shape, arrangement, size and material of the firstelastic connecting portions 4, 4 and the second elastic connectingportions 5, 5 to those as described above, the torsional rigidity of thefirst elastic connecting portion 4 with respect to the rotation centralaxis 41 can be set to a predetermined value larger than that of thesecond elastic connecting portion 5. This makes it possible to enlargethe rotational angle (deflection angle) of the movable portion 2 whilepreventing the driving portion 1 from vibrating.

Further, since each of the first elastic connecting portions 4, 4 isconstructed from the two connecting members 42, 42, it is possible toreduce the stress generated at the contact portion between each of theconnecting members 42, 42 and each of the driving portions 1, 11compared with the case where each of the first elastic connectingportions 4, 4 is constructed from one connecting member 42. This makesit possible to prevent the elastic deformation of each of the drivingportions 1, 11 surely. Therefore, it is possible to reduce or preventany vibration other than the torsional vibration of the driving portions1, 11, whereby the rotational operation thereof can be carried outsmoothly.

In addition, by mainly generating the bending deformation of each of theconnecting members 42, 42 when driving the driving portions 1, 11, it isalso possible to obtain the following effects.

(1) Since the elastic deformation of each of the driving portions 1, 11is prevented more surely, it is possible to carry out the rotationaloperation of the driving portions 1, 11 more smoothly. This makes itpossible to obtain an actuator 100 having very good following capability(response) of the movable portion 2 with respect to the driving of thedriving portions 1, 11 and excellent vibration characteristic(behavioral property).

(2) Since the torsional rigidity of the first elastic connecting portion4 can be enlarged further compared with the case where the torsionaldeformation is generated in each of the connecting members 42, 42, it ispossible to enlarge the rotational angle (deflection angle) of themovable portion 2 further.

In this regard, the length (distance) between the rotation central axis41 on the driving portion 1 and one end portion 12 of the drivingportion 1 in a direction substantially perpendicular to the rotationcentral axis 41 is defined as L1, the length (distance) between therotation central axis 41 on the driving portion 11 and an end portion 12of the driving portion 11 in a direction substantially perpendicular tothe rotation central axis 41 is defined as L2, and the length (distance)between the rotation central axis 41 on the movable portion 2 and an endportion 13 of the movable portion 2 in a direction substantiallyperpendicular to the rotation central axis 41 is defined as L3. Sincethe driving portions 1, 11 are provided independently of each other inthe present embodiment, the driving portions 1, 11 do not interfere inthe movable portion 2. Thus, it is possible to make the lengths L1 andL2 smaller regardless of the size of the movable portion 2 (that is, thelength L3). This makes it possible to enlarge the rotational angle(deflection angle) of each of the driving portions 1, 11, and thereforeit is possible to enlarge the rotational angle of the movable portion 2.

Further, by making the lengths L1 and L2 smaller, it is possible to makethe distance between the respective driving portions 1, 11 and each ofthe corresponding electrodes 7 smaller. This makes it possible toenlarge the electrostatic force, and therefore it is possible todiminish the alternating voltage applied between each of the drivingportions 1, 11 and each of the electrodes 7. In this regard, it ispreferable that the lengths L1, L2 and L3 (that is, sizes of the drivingportions 1, 11 and the movable portion 2) are set so as to satisfy therelations: L1<L3 and L2<L3.

By satisfying the relations described above, it is possible to make thelengths L1 and L2 further smaller. This makes it possible to enlarge therotational angles of the driving portions 1, 11, and therefore it ispossible to further enlarge the rotational angle (deflection angle) ofthe movable portion 2. In this case, it is preferable that the maximumdeflection angle of the movable portion 2 is set so as to become 20° ormore.

Moreover, by making the lengths L1, L2 smaller in this manner, it ispossible to further reduce the distance between each of the drivingportions 1, 11 and each of the corresponding electrodes 7, and thereforeit is possible to further diminish the alternating voltage appliedbetween each of the driving portions 1, 11 and each of the correspondingelectrodes 7. Therefore, it is possible to realize (achieve) thelow-voltage driving for the driving portions 1, 11 and the displacement(rotation) of the movable portion 2 with a large rotational angle.

For example, in the case where such an actuator 100 described above isapplied to an optical scanner used in apparatuses such as laser printer,confocal scanning laser microscope, it is possible to make the apparatussmaller more easily.

In this regard, as mentioned above, although the lengths L1 and L2 areset so as to have substantially the same size in the present embodiment,it is no wonder that the length L1 may be different from the length L2.

It should be noted that such a two-degree-of-freedom vibration typeactuator 100 has a frequency characteristic as shown in FIG. 5 betweenthe amplitudes (deflection angles) of the driving portions 1, 11 and themovable portion 2 and the frequency of the applied alternating voltage.Namely, the two-degree-of-freedom vibration system constituted from thefirst vibration system of the driving portions 1, 11 and the firstelastic connecting portions 4, 4, and the second vibration system of themovable portion 2 and the second elastic connecting portions 5, 5 hastwo resonance frequencies fm₁ (kHz) and fm₃ (kHz) (where, fm₁<fm₃) atwhich the amplitudes of the driving portions 1, 11 and the movableportion 2 become large, and one antiresonance frequency fm₂ (kHz) atwhich the amplitude of the driving portions 1, 11 becomes substantiallyzero.

In this actuator 100, it is preferable that the frequency F of analternating voltage to be applied between each of the driving portions1, 11 and the corresponding electrodes 7 is set so as to besubstantially the same as a lower resonance frequency of the tworesonance frequencies, that is, the frequency F is set so as to besubstantially the same as fm₁. By setting the frequency F (kHz) of thealternating voltage to be applied so as to be substantially the same asfm₁ (kHz), it is possible to increase the rotational angle (deflectionangle) of the movable portion 2 while the vibration of the drivingportions 1, 11 is suppressed. In this regard, it is to be noted that, inthis specification, the fact that F (kHz) is substantially the same asfm₁ (kHz) means that F and fm₁ satisfy the relation: (fm₁−1)≦F≦(fm₁+1)

It is preferable that the average thickness of each of the drivingportion 1, 11 is in the range of 1 to 1,500 μm, and more preferably itis in the range of 10 to 300 μm. Similarly, it is preferable that theaverage thickness of the movable portion 2 is in the range of 1 to 1,500μm, and more preferably it is in the range of 10 to 300 μm.

Further, it is preferable that the spring constant k₁ of each of thefirst elastic connecting portions 4, 4 is in the range of 1×10⁻⁴ to1×10⁴ Nm/rad, and more preferably it is in the range of 1×10⁻² to 1×10³Nm/rad. Further more preferably it is in the range of 1×10⁻¹ to 1×10²Nm/rad. Similarly, it is preferable that the spring constant k₂ of eachof the second elastic connecting portions 5, 5 is in the range of 1×10⁻⁴to 1×10⁴ Nm/rad, and more preferably it is in the range of 1×10⁻² to1×10³ Nm/rad. Further more preferably it is in the range of 1×10⁻¹ to1×10² Nm/rad. By setting the spring constant k₁ of each of the firstelastic connecting portions 4, 4 and the spring constant k₂ of each ofthe second elastic connecting portions 5, 5 to predetermined valueswithin the range given above, it is possible to further increase therotational angle (deflection angle) of the movable portion 2 while thevibration of each of the driving portions 1, 11 is suppressed.

Moreover, it is preferable that, in the case where the spring constantof each of the first elastic connecting portions 4, 4 is defined as k₁,and the spring constant of each of the second elastic connectingportions 5, 5 is defined as k₂, then k₁ and k₂ satisfy the relation:k₁>k₂. This makes it possible to further increase the rotational angle(deflection angle) of the movable portion 2 while the vibration of eachof the driving portions 1, 11 is suppressed.

In this regard, the values of the torsional rigidity, the second momentof area and the spring constant k₁ of each of the first elasticconnecting portions 4, 4 are calculated by assuming that the twoconnecting members 42, 42 are formed in an integrated manner.

Furthermore, it is preferred that, in the case where the moment ofinertia of each of the driving portions 1, 11 is defined as J₁ and themoment of inertia of the movable portion 2 is defined as J₂, then J₁ andJ₂ satisfy the relation: J₁<J₂, and more preferably they satisfy therelation: J₁<J₂. This makes it possible to further enlarge therotational angle (deflection angle) of the movable portion 2 while thevibration of each of the driving portions 1, 11 is suppressed.

Now, the natural frequency ω₁ of the first vibration system constitutedfrom the driving portions 1, 11 and the first elastic connectingportions 4, 4 can be determined by the formula: ω₁=(k₁/J₁)^(1/2) in thecase where J₁ represents the moment of inertia of each of the drivingportions 1, 11 and k₁ represents the spring constant of each of thefirst elastic connecting portions 4, 4. Similarly, the natural frequencyω₂ of the second vibration system constituted from the movable portion 2and the second elastic connecting portions 5, 5 can be determined by theformula: ω₂(k₂/J₂)^(1/2) in the case where J₂ represents the moment ofinertia of the movable portion 2, and k₂ represents the spring constantof each of the second elastic connecting portions 5, 5.

Further, it is preferable that the natural frequency ω₁ of the firstvibration system and the natural frequency ω₂ of the second vibrationsystem determined in such a manner described above satisfy the relation:ω₂>ω₂. This makes it possible to further increase the rotational angle(deflection angle) of the movable portion 2 while the vibration of eachof the driving portions 1, 11 is suppressed.

In this regard, in the actuator 100 of the present embodiment, it ispreferred that the actuator 100 has a piezoresistive element in at leastone of the pair of first elastic connecting portions 4, 4 and the pairof second elastic connecting portions 5, 5 thereof. This makes itpossible to detect rotational angles and rotation frequencies, forexample. Further, it is also possible to utilize the detection resultsto control the posture of the movable portion 2.

Such an actuator 100 can be manufactured as follows, for example.

FIG. 6 is a drawing (longitudinal sectional view) for explaining amethod of manufacturing the actuator 100 according to the presentembodiment. In the following description using FIG. 6, for convenienceof description, it is to be noted that the upper side and the lower sidein FIG. 6 will be referred to as the “upper side” and “lower side”,respectively.

<A1> First, as shown in FIG. 6A, a silicon substrate (first basematerial) 30 is prepared.

Next, photoresist is applied onto the upper surface of the siliconsubstrate 30, and then exposure and development are carried out for thesilicon substrate 30 with the photoresist. Thus, as shown in FIG. 6B, aresist mask 23 is formed so as to correspond to the shape of thesupporting portions 3, 3.

The surface of the silicon substrate 30 with the resist mask 32 issubjected to an etching process, and then the resist mask 32 iseliminated. Thus, as shown in FIG. 6C, a concave portion 300 is formedat a region other than the portions corresponding to the supportingportions 3, 3.

As for the etching method, for example, various physical etching methodssuch as a plasma etching method, a reactive ion etching method, a beametching method, and a photo-assist etching method, and various chemicaletching methods such as a wet etching method may be mentioned. In thiscase, one kind of etching method or two or more kinds of etching methodsamong them may be utilized.

In this regard, the similar etching method or methods can be utilized ineach of the etching processes will be described later.

Next, photoresist is applied onto the upper surface of the siliconsubstrate 30 again, and exposure and development are then carried outfor the silicon substrate 30 with the photoresist. Thus, as shown inFIG. 6D, a resist mask 33 is formed so as to correspond to the shape ofthe driving portions 1, 11, the movable portion 2, the supportingportions 3, 3, the first elastic connecting portions 4, 4 and the secondelastic connecting portions 5, 5 when the silicon substrate 30 is viewedfrom the lower side in FIG. 2.

The surface of the silicon substrate 30 with the resist mask 33 issubjected to an etching process until the concave portions formed by theetching process pass through the silicon substrate 30, and then theresist mask 33 is eliminated. Thus, as shown in FIG. 6E, a structure 50in which the driving portions 1, 11, the movable portion 2, thesupporting portions 3, 3, the first elastic connecting portions 4, 4 andthe second elastic connecting portions 5, 5 are formed is obtained.

A metal film is then formed on the movable portion 2 to form a lightreflecting portion thereon.

As for the method of forming the metal film, various chemical vapordeposition (CVD) methods such as a plasma CVD method, a thermal CVDmethod, and a later CVD method, various dry plating methods such as avacuum evaporation method, a sputtering method (a low-temperaturesputtering method), and an ion plating method, various wet platingmethods such as an electrolytic plating method, and an electrolessplating method, a spray method, a sol-gel method, a metal organicdeposition (MOD) method and joint of a metal foil may be mentioned.

In this regard, the similar method of forming the metal film can beutilized in each of the metal film forming method will be describedlater.

<A2> Next, as shown in FIG. 6F, a silicon substrate 60 is prepared forforming the counter substrate 6.

A metal mask made of aluminum or the like is formed on one surface ofthe silicon substrate 60 at the portion other than the region where anopening 61 is to be formed.

Next, the one surface of the silicon substrate 60 with the metal mask issubjected to an etching process until a concave portions formed by theetching process pass through the silicon substrate 60, and then themetal mask is eliminated. Thus, as shown in FIG. 6G, the countersubstrate 6 in which the opening 61 has been formed is obtained.

Next, as shown in FIG. 6H, four electrodes 7 are formed on the countersubstrate 6. The electrodes 7 can be formed by forming a metal film anda mask corresponding to the positions of the electrodes 7 on the surfaceof the counter substrate 6 in which the opening 61 has been formed, andeliminating the mask after subjecting the counter substrate 6 with themask to an etching process.

In this regard, the electrodes 7 may be formed prior to the formation ofthe opening 61.

<A3> Next, as shown in FIG. 6I, the supporting portions 3, 3 of thestructure 50 obtained at the step <A1> described above are bonded to thecounter substrate 6 obtained at the step <A2> by means of, for example,direct bonding, anodic bonding in which a glass material containingsodium ion (Na⁺) is used, or the like.

In the case where a glass substrate is used as a substrate for formingthe counter substrate 6, in order to form the opening 61, for example,shot blast, sand blast, laser beam processing or the like can be used inaddition to the etching methods as described above. Further, in thiscase, in order to bond the counter substrate 6 to the structure 50,anodic bonding or the like can be used, for example.

As described above, the actuator 100 of the present embodiment ismanufactured.

In this regard, although, in this method of forming the actuator 100 ofthe present embodiment, the driving portions 1, 11, the movable portion2, the supporting portions 3, 3, the first elastic connecting portions4, 4 and the second elastic connecting portions 5, 5 are integrallyformed by subjecting the silicon substrate 30 to the etching processfrom the side of the concave portion 300 thereof, the method of formingthe actuator 100 is not limited thereto. For example, the drivingportions 1, 11, the movable portion 2, the supporting portions 3, 3, thefirst elastic connecting portions 4, 4 and the second elastic connectingportions 5, 5 may be integrally formed by subjecting the siliconsubstrate 30 to the etching process from the side opposite to the sideof the concave portion 300 thereof.

The actuators 100 described above based on the preferred embodiment canbe preferably applied to optical scanners to be used in laser printers,bar-code readers, confocal scanning laser microscopes and the like, ordisplays for imaging, for example.

Although the actuator 100 according to the invention has been descriedwith reference to the embodiment shown in the drawings, the invention isnot limited thereto.

Further, so long as the same or similar functions are achieved, it ispossible to make various changes and additions to each portion of theactuator 100 of the invention.

Moreover, in the embodiment described above, even though it has beendescribed that the light reflecting portion 21 is provided on onesurface of the movable portion 2, in the actuator 100 according to theinvention, the light reflecting portion 21 may be provided on theopposite surface of the movable portion 2, or may be provided on bothsurfaces of the movable portion 2, for example.

Furthermore, in the actuator 100 of the embodiment described above, eventhough it has been described that the electrodes 7 are provided on thecounter substrate 6, in the actuator 100 according to the invention, theelectrodes 7 may be provided on the driving portion 1, or may beprovided on both of the counter substrate 6 and the driving portion 1,respectively.

Further, in the actuator 100 of the embodiment described above, eventhough it has been described that the two pairs of electrodes 7 arerespectively provided at the positions corresponding to the drivingportions 1, 11, the actuator 100 according to the invention is notlimited thereto. For example, one or three or more electrode 7 may beprovided at each of the corresponding positions.

In this regard, in the case where one electrode 7 is provided at each ofthe positions corresponding to the driving portions 1, 11, it ispreferable that sinusoidal wave (alternating voltage) to which an offsetvoltage is added and in which the minimum electric potential becomesground potential is applied to the one electrode 7, for example.

Moreover, in the actuator 100 of the embodiment described above, eventhough it has been described that the insulating film is provided on thesurface of each of the driving portions 1, 11, which is a surface facingthe electrodes 7, for preventing a short circuit from occurring, in theactuator 100 according to the invention, the insulating film may beprovided on the surface of each of the electrodes 7 or may be providedon the surfaces of both of the driving portions 1, 11 and the electrodes7, for example.

Furthermore, in the actuator 100 of the embodiment described above, eventhough it has been described that the pair of driving portions 1, 11 areprovided, in the actuator 100 according to the invention, one drivingportion may be provided so as to enclose the movable portion, forexample.

Further, in the actuator 100 of the embodiment described above, thecharacteristic of each of the first elastic connecting portions 4, 4 maybe mainly bending deformation (that is, the first elastic connectingportions 4, 4 may have a bending characteristic). For example, the firstelastic connecting portions 4, 4 may have both characteristics of thebending deformation and the torsional deformation.

Moreover, so long as the torsional rigidity of the first elasticconnecting portion 4 is set to a predetermined value larger than that ofthe second elastic connecting portion 5, the shape, arrangement, sizeand material of the first elastic connecting portion 4 is not limited toone shown in the drawings (one described above).

FIG. 7 is a plan view which shows an alternative example of the firstelastic connecting portion. As for the structure of the first elasticconnecting portion 4 in which the torsional rigidity thereof is largerthan that of the second elastic portion 5, the following cases (a) to(j) may be mentioned as examples.

Namely, such cases includes: (a) the case where the width a of theconnecting member 42 (first elastic connecting portion 4) is wider thanthe width b of the second elastic connecting portion 5; (b) the casewhere the length e of the connecting member 42 is shorter than thelength f of the second elastic connecting portion 5; (c) the case whereone or more through-hole is formed in the connecting member 42; (d) thecase where two connecting members 42 are extendedly provided toward thedriving portion 1 (or 11) from both side surfaces of a main connectingmember 42 coaxially provided with respect to the rotation central axis41; (e) the case where two or more connecting members 42 (two in FIG.7E) are arranged at each of the both sides of the rotation central axis41; (f) the case where one end of each of the connecting members 42 ispositioned at the opposite side to the other end thereof with respect tothe rotation central axis 41 (two connecting members 42 are provided soas to intersect with each other and the cross-point is provided on therotation central axis 41); (g) the case where a plurality pair ofconnecting members 42 are formed so that a pair of connecting members 42therein intersect with each other; (h) and (i) the case where a part orthe whole of each of the connecting members 42 are curved or bent; and(j) the case where one connecting member 42 has a substantially H shapewhen viewed from the top of the actuator 100. In this regard, one or twoor more structure among the cases (a) to (j) may be combined as theconnecting members 42.

1. An actuator comprising: a pair of driving portions spaced apart fromeach other; a movable portion provided between the pair of drivingportions; a pair of supporting portions for supporting the pair ofdriving portions and the movable portion; a pair of first elasticconnecting portions which respectively connect the pair of drivingportions to the pair of supporting portions so that each of the drivingportions can rotate with respect to the supporting portions; and a pairof second elastic connecting portions which respectively connect themovable portion to the pair of driving portions so that the movableportion can rotate in accordance with the rotation of the pair ofdriving portions, wherein each of the pair of driving portions rotatesaround a rotation central axis of the actuator by application of analternating voltage, whereby the movable portion rotates around therotation central axis of the actuator, and wherein the torsionalrigidity of each of the first elastic connecting portions is larger thanthat of each of the second elastic connecting portions.
 2. The actuatoras claimed in claim 1, wherein the second moment of area of each of thepair of first elastic connecting portions is larger than that of each ofthe second elastic connecting portions.
 3. The actuator as claimed inclaim 1, wherein each of the pair of first elastic connecting portionsis constructed from two connecting members each spaced from the rotationcentral axis with a predetermined distance.
 4. The actuator as claimedin claim 3, wherein each of the pair of driving portions is constructedso as to rotate around the rotation central axis by making each of thetwo connecting members bending deformation.
 5. The actuator as claimedin claim 1, wherein each of the pair of second elastic connectingportions is coaxially provided with respect to the rotation centralaxis.
 6. The actuator as claimed in claim 1, wherein each of the pair ofdriving portions is driven by means of Coulomb force generated by theapplication of the alternating voltage.
 7. The actuator as claimed inclaim 1, further comprising: a counter substrate provided so as to bespaced apart from the pair of driving portions with a predetermineddistance; and a plurality of electrodes respectively provided at thepositions on the counter substrate corresponding to those of the pair ofdriving portions, wherein each of the pair of driving portions is drivenby means of Coulomb force generated by applying the alternating voltagebetween each of the plurality of electrodes and the correspondingdriving portion.
 8. The actuator as claimed in claim 7, wherein theplurality of electrodes include two pairs of electrodes, and each pairof the two pairs of electrodes is provided at the positions on thecounter substrate which correspond to both ends of the correspondingdriving portion in the direction substantially perpendicular to therotation central axis.
 9. The actuator as claimed in claim 7, whereinthe counter substrate has an opening at the portion corresponding to themovable portion.
 10. The actuator as claimed in claim 1, wherein thepair of supporting portions, the movable portion, the pair of drivingportions, the pair of first elastic connecting portions and the pair ofsecond elastic connecting portions are formed by subjecting a basematerial to an etching process.
 11. The actuator as claimed in claim 10,wherein the base material is formed of silicon.
 12. The actuator asclaimed in claim 1, further comprising a light reflecting portionprovided on the movable portion.
 13. The actuator as claimed in claim 1,wherein the actuator is of the type which employs atwo-degree-of-freedom vibration system, and the frequency of thealternating voltage is set so as to be substantially the same as a lowerresonance frequency of resonance frequencies of thetwo-degree-of-freedom vibration system at which the pair of drivingportions and the movable portion resonate.
 14. The actuator as claimedin claim 1, wherein at least one of the pair of first elastic connectingportions and the pair of second elastic connecting portions includes apiezoresistive element.